This invention relates generally to inkjet printers. More particularly, the invention relates to a technique for improving the quality of inkjet printing systems by introducing a controlled variance in a scan axis to thus hide vertical bands that may form during the printing process.
U.S. application Ser. No. 09/199,882, filed Nov. 24, 1998, entitled xe2x80x9cAlignment of Ink Dots in an Inkjet Printer,xe2x80x9d by Paul D. Gast et al., is assigned to the present assignee and incorporated herein by reference in its entirety. That application discloses a technique for compensating for misalignment that may occur during printing operations. In this respect, according to the disclosed technique, a test pattern is printed to determine whether any compensation for misalignment is required during the printing of a plot. Offset errors are introduced during the printing of the plot to compensate for misalignment. Thus, that application pertains to the reduction of misalignment that may occur during printing and thus is not concerned with the reduction of vertical banding.
FIG. 1 is a simplified example of a conventional inkjet printer 10. This example will be used to illustrate some of the problems associated with known ink jet printers. As illustrated in FIG. 1, conventional ink jet printer 10 includes an input tray 12 containing sheets of print medium (e.g., paper) 14 which pass through a print zone 15 for being printed upon. The print medium 14 is then forwarded to an output tray 16. A movable carriage 20 holds print cartridges 22, 24, 26, and 28, which may hold various colored inks, in addition to black ink. The carriage 20 is traversed along a scan axis by a belt and pulley system and slides along slide rod 30.
In use, printing signals from an external computer (not shown) are processed by inkjet printer 10 to generate a bitmap of the dots to be printed. The bitmap is then converted into firing signals for the printhead. The position of the carriage 20 as it traverses back and forth along the scan axis is determined from an optical encoder strip 32, detected by a photoelectric element on carriage 20, to cause the various ink ejection elements on each printer cartridge to be selectively fired at the proper time during the carriage scan.
FIG. 2 illustrates the printhead portion of a print cartridge, such as print cartridge 22 in FIG. 1, while FIG. 3 is a top down detailed view of a nozzle plate 34 on the print cartridge. Three hundred nozzles 35 arranged in two vertical rows 38 are shown in FIG. 3. The primitives P1-P14 are labeled on the nozzle plate 34. The print cartridge 22 has contact pads 36 formed on a circuit which electrically contact electrodes in cartridge 20 for receiving power and ground signals as well as the firing signals for the various ink ejection elements.
FIG. 4 illustrates a portion of the printhead substrate, underneath nozzle plate 34, associated with a single primitive. The printhead substrate is typically a rectangular piece of silicon having formed on it ink channels 40, ink ejection chambers 42, and heater resistors 44 using photolithographic techniques. The various ink channels 40 and chambers 42 are formed by a barrier layer 45 of photoresist. Each chamber 42 is operable to receive ink via an associated ink channel 40. When current passes through a heater resistor 44, the received ink is vaporized to cause a droplet of ink to be ejected by an associated nozzle 35.
To accurately print onto a print medium, the printheads of the ink jet printer devices must be accurately positioned over that portion of the print medium to which it is to print. In addition, the heater resistor 44 must be fired at the correct moment as the printhead is moved along the scan axis. Small inaccuracies due to uncontrolled movements, oscillations, etc., may allow for any faults in the printing output to become visible. Moreover, because the printheads are generally mounted in a mechanical part that moves over the print medium, errors occurring due to the printhead movements over the print medium may additionally adversely affect the accuracy and thus the quality of the printed output.
One manner in which conventional ink jet printer devices attempt to address the above-stated inaccuracies is to utilize a multi-pass printing process. In a multi-pass printing process, each part of the printing output is printed using a different part of the printhead. That is, with reference to FIG. 3, each part of the printed output for a given pass is printed with a different nozzle 35. As illustrated in FIG. 5, the multi-pass printing method makes multiple passes 46-52, with the print medium making small advancing movements in the direction 54 between the passes, to generate the desired images. Thus, a desired image is printed with the printhead making a first pass 46 in a scan axis, a second pass 48 in a scan axis, etc. By virtue of the multi-pass printing technique, those areas in which malfunctioning nozzles are to print may be printed upon by at least one functioning nozzle.
However, the multi-pass printing method is not completely immune from defects which may occur during a printing process. By virtue of a plurality of factors, the directionality and placement of the nozzles may become somewhat skewed. For example, if there are any sudden changes in the friction force or defects in the surfaces over which the carriage 20 runs, some oscillations or mechanical shifts may give rise to directionality and placement errors. Additionally, the electrical cables that connect the carriage electronics with external electronics may change the electrical properties (e.g., RC time constants) when the carriage 20 runs along the scan axis. This changes the time delay of the signal transmitted through the cables, thus potentially shifting the time for firing the drops of ink and may give rise to placement errors. In this respect, the placement errors may be effectuated each time the printhead 22-28 makes a pass to print an image. Thus, as illustrated in FIG. 6, this may result in the formation of vertical bands 56 during a printing operation which implements the multi-pass process.
The vertical bands 56 illustrated in FIG. 6 may result from the row of nozzles 38 being fired simultaneously. As illustrated in FIG. 7, reference numeral 38 represents a row of nozzles which are configured to travel in a scan axis direction 58. When there is an error 60, e.g., a fly time delay, mechanical shift, or the like, each of the nozzles 35 in the row of 38 typically fire ink drops 62 some time after the fire signal 64 is received by the printhead 22-28 indicated by arrow 66. In this instance, a vertical row of ink drops may be fired according to the error 60 each time the printhead makes a pass across the scan axis. This may result in the formation of the vertical bands 56 throughout the printed image 55 as illustrated in FIG. 8.
Accordingly, known multi-pass printing methods have been relatively inadequate to print images on print media without vertical bands when errors occur and therefore, known multi-pass printing methods suffer from a variety of drawbacks and disadvantages.
According to one aspect, the present invention pertains to a method of reducing vertical banding in a multi-pass printing process. The multi-pass printing process utilizes a printhead having a plurality of nozzles positioned in a substantially non-linear arrangement along a surface of the printhead. The nozzles are operable to fire an ink drop within a predetermined time in response to receipt of a fire signal during each pass of said multi-pass printing process. In the method, a controlled variance is introduced to vary the predetermined time of firing of the ink drop. The controlled variance is configured to vary the predetermined time of firing the ink drop for each pass of the multi-pass printing process.
In accordance with another aspect, the present invention relates to a method of operating a printer device to print onto a print medium. In the method, a first firing sequence is signaled to a first set of nozzles in the printer device to fire an ink drop during a first pass of the nozzles over a first portion of the print medium. The first set of nozzles are operable to fire an ink drop within a predetermined time in response to receipt of the first firing sequence signal. A first controlled variance is introduced to vary the predetermined time of firing of the ink drop in response to receipt of the first firing sequence signal. A second firing sequence is signaled to a second set of nozzles in the printer device to fire an ink drop during a second pass of the nozzles over a second portion of the print medium. The second set of nozzles are operable to fire an ink drop within a predetermined time in response to receipt of the second firing sequence signal. Additionally, a second controlled variance is introduced to vary the predetermined time of firing of the ink drop in response to receipt of the second firing sequence signal.
In accordance with another aspect, the present invention relates to an ink jet printer device configured to print an image by utilizing a multi-pass printing process. The device includes a means for firing an ink drop at a predetermined time following receipt of a fire signal. A means for varying the predetermined time of firing the ink drop in response to receipt of the fire signal, in which the predetermined time of firing the ink drop varies for each pass in the multi-pass printing process.
In comparison to known printing methods, certain embodiments of the present invention are capable of achieving certain advantages, such as, substantially reducing the formation of vertical bands which may occur during printing processes.